In two-stroke engines, scavenging passages that open to the crank chamber formed inside the crankcase extend as smoothly curved passages inside the crankcase to the fastening surfaces of the crankcase and the cylinder, through the scavenging passages, and connect to scavenging ports provided in the cylinder.
FIG. 8 is an exploded perspective view of a crankcase and a cylinder of a conventional two-stroke engine. FIG. 9 is a cross section along F-F of FIG. 8, which is a cross-sectional view of the crankcase of the conventional two-stroke engine as viewed from a direction orthogonal to the crankshaft axis. FIG. 10 is a diagram as viewed in the direction of arrows G-G in FIG. 9, which is a top view of the crankcase. FIG. 11 is a cross section as viewed in the direction of arrows H-H in FIG. 10.
In FIG. 8 to FIG. 11, reference numeral 101 denotes a cylinder; a combustion chamber is formed inside the cylinder 101. There are provided a piston 130 moving up and down inside the cylinder 101, and a crankshaft 108 for converting the up and down movement of the piston 130 into rotational force.
Reference numeral 102 denotes the crankcase, inside which a crank chamber 106 is formed. Reference numeral 140 denotes two scavenging ports provided in side portions of the cylinder 101 to face opposite each other, FIG. 8 showing only one of these. The scavenging ports 140 are each communicated with the crank chamber 106 via scavenging passages 104b inside the cylinder 101, scavenging passages 104a formed in a curved shape in both side walls of the crankcase 102, and a pair of scavenging passage inlets 104c. 
Reference numeral 118 denotes air passages for supplying leading air, connected midway of the scavenging passages 104b so that leading air from an air cleaner is supplied to the scavenging ports 140 through the air passages 118 and scavenging passages 104b. 
Reference numeral 132 denotes an insulator for thermally insulating the intake system from the engine body, the insulator 132 being fastened to a side face of the cylinder 2 with bolts. An upper passage 132a inside the insulator 132 forms an air passage for supplying leading air. This air passage is connected midway of the scavenging passages 104b, so that leading air is supplied to the scavenging ports 140 via the air passages 118 and scavenging passages 104b. A lower passage 132b inside the insulator 132 forms an air/fuel mixture passage for supplying an air/fuel mixture into the crank chamber 106. This air/fuel mixture passage communicates with inside of the cylinder 101 via an intake port 142.
In FIG. 8, the crankcase 102 is made up of a front crankcase 102a and a rear crankcase 102b that are front and back parts divided at a front and back split surface 125 orthogonal to the crankshaft axis 180. After the crankshaft 108 and others have been assembled inside, the front crankcase 102a and rear crankcase 102b are fastened together and united, with a plurality of bolts using bolt holes 126.
Reference numeral 103 denotes a flat and smooth cylinder mounting surface formed at the top of the crankcase 102. The cylinder 101 is fastened with a plurality of bolts 114, with its lower face 103′ making contact with this cylinder mounting surface 103.
The scavenging passages 104a and the scavenging passage inlets 104c are each formed inside the front crankcase 102a and rear crankcase 102b symmetrically about the front and back split surface 125. The upper ends of the scavenging passages 104a are open in the cylinder mounting surface 103.
Reference numeral 110 denotes guide members, inserted from the openings in the cylinder mounting surface 103, their upper faces forming part of the cylinder mounting surface 103. The guide members 110 are formed with passage surfaces that smoothly connect to the scavenging passages 104a. The guide members 110 are provided with protrusions, for example, in their upper faces and sides, while grooves are cut in the crankcase 102, so that the guide members are fastened to the crankcase 102 with these protrusions fitted in the grooves.
In the two-stroke engine equipped with the scavenging passage structure configured as shown in FIG. 8 to FIG. 11, the air/fuel mixture from the crank chamber 106 formed inside the crankcase 102 is introduced into the scavenging passages 104a partly formed by the guide members 110 having passage surfaces formed as smooth curved surfaces, via the scavenging passage inlets 104c, and flows through the scavenging passages 104a formed as smoothly curved passages, and through the scavenging passage 104b formed in the cylinder, to be supplied to the scavenging ports 140. The air/fuel mixture is thus supplied as a smooth and powerful flow into the scavenging ports 140 without any flow loss such as a flow rate reduction, as the mixture flows through the scavenging passages 104a formed as smoothly curved passages without turns at right angles or the like.
The conventional scavenging passage structure for a two-stroke engine using the guide members described using FIG. 8 to FIG. 11 is disclosed in Patent Document 1.